The Choralis Constantinus is a collection of over 375 Gregorian chant-based polyphonic motets for the proper of the mass composed by Heinrich Isaac and his pupil Ludwig Senfl.
Isaac was in Constance at the time (April 1508) with the Imperial court as Maximilian had called a meeting of the German nobility (Reichstag) there.
After Isaac's death in 1517 his pupil Ludwig Senfl, who had been a member of the Imperial chapel choir, compiled music composed for Constance and for the Habsburg Imperial court into the collection which was published by Hieronymus Formschneider in Nuremberg in three volumes (1550–1555) and titled the Choralis Constantinus.
Each Feast contains a polyphonic motet, based on the corresponding Gregorian chant for the Introit, Alleluia (or Tract) and Communion.
Those Propers all include alternate verses of the Sequence hymn, set thus so as to allow for alternatim performance between the choir and the organist.